The correct answer is: alternative.
An alternative is a different option or course of action. In this case, if our friends are not able to take us to the airport in their car, we must make alternative arrangements to get there. This could include taking a taxi, public transportation, or asking another friend or family member for a ride.
The other options are not as accurate. Another means “one more of the same kind.” In this case, we are not looking for one more way to get to the airport, we are looking for a different way. Alternate means “occurring or existing at intervals.” In this case, we are not looking for a way to get to the airport that occurs at intervals, we are looking for a different way. Possible means “able to be done or achieved.” In this case, we are not looking for a way to get to the airport that is able to be done or achieved, we are looking for a different way.